Recently, several medium vinyl polybutadienes (MVPBd) have been offered commercially. Because these polybutadienes have about 30 to 40% 1,2-polybutadiene (vinyl) microstructure, they have a glass transition temperature (T.sub.g) of approximately -70.degree. C. This T.sub.g is close to that of emulsion SBR. Likewise, the behavior of the MVPBd's is similar to that of emulsion SBR when compounded and cured.
The purpose of this invention is to outline an improved process for making a novel MVPBd which could compete with the commercial MVPBd's as well as emulsion SBR.
Since MVPBd requires no styrene, its cost and availability is independent of styrene. Should the benzene (and hence styrene) shortage occur again or the price differential between butadiene and styrene increase, MVPBd would offer a relatively inexpensive, available replacement for either solution or emulsion SBR.
The use of polar catalyst modifier with alkyl-lithium has been investigated. (T A Antkowiak, A E Oberster, A F Halasa and D P Tate, J. Poly. Sci., A-1, 10, 1319; 1972.) It is known that the vinyl content of the MVPBd is proportional to the ratio of polar catalyst modifier to active alkyl-lithium and inversely proportional to the reaction temperature at constant catalyst and modifier level.
However, detailed reports on the subject make no mention of the possibility of using more than a single catalyst modifier or the possible results of doing so.
A well known characteristic of virtually all anionic polymerizations conducted with alkyl-lithium catalysts is the low heterogeneity index (H.sub.i) of the resulting polymer. This is true for the MVPBds also. These relatively monodisperse polymers are uniformly poor in processing. As a result, a variety of techniques have been employed to improve the processability of the MVPBds.
One of these techniques involves the production of block polymers of the form A-B wherein the A-block is polybutadiene having a high (about 90%) 1,4-polybutadiene microstructure and the B-block is high enough (about 70%) in 1,2 or vinyl content so that the resulting polymer has a net vinyl content of 40 to 50%.
The simplest approach to improving the processability of the polymer via this technique involves polymerizing the butadiene to about 50% conversion in the absence of any polar catalyst modifiers and then adding a relatively large amount of some material such as tetrahydrofuran, glyme or diglyme. (Brit. Pat. No. 1,231,657, May 12, 1971 and U.S. Pat. No. 3,301,840, Jan. 31, 1967. )
Alternatively, that segment of the block polymer having a high vinyl content may be produced first using a catalyst modifier such as N,N,N',N-tetramethylethylene diamine (TMEDA) which is then reduced in its activity by adding, at about 50% conversion, a complexing agent such as diethyl-zinc (U.S. Pat. No. 3,830,880, Aug. 20, 1974.) Another approach to the same product involves allowing the butadiene to completely polymerize using an unmodified alkyl-lithium catalyst and then adding fresh monomer and a polar catalyst modifier to the still "living" reaction. (U.S. Pat. No. 3,140,278, July 7, 1964.)
Another MVPBd which also contains about 40% vinyl polybutadiene differs in structure in that the vinyl-polybutadiene is not uniformly distributed throughout the polymer chain. That is, the vinyl content increases gradually from one end of the polymer chain to the other rather than radically as in the A-B block MVPBds. These are termed "tapered" polymers. Since the effectiveness of the polar catalyst modifier is inversely proportional to the reaction temperature, it is possible to produce a tapered MVPBd by simply allowing the reaction to exotherm. (Brit. Pat. No. 1,320,945 June 20, 1973 and U.S. Pat. No. 3,829,409, Aug. 13, 1974.) The improved processability of this polymer might be explained by the branching through alkylation which would be expected at the final elevated temperatures (125.degree. C.=247.degree. F.) reported by patentees for their process.
This same phenomenon has been exploited for high 1,4-polybutadiene where a temperature of 315.degree. F. (157.degree. C.) is reported. (U.S. Pat. No. 3,629,223, Dec. 21, 1971.) Since the polar catalyst modifiers increase the reactivity of the polybutadienyl-lithium chain ends, it would be expected that alkylation (branching) could occur at the lower temperatures reported in U.S. Pat. No. 3,829,409.